The Idlib region of some three million people is supposed to be protected by a September buffer zone deal, but the jihadist-run enclave has come under mounting bombardment by the government and its ally Russia since late April.

The two children were killed in a garage on the edge of the town of Maaret al-Numan, the Syrian Observatory for Human Rights said.

The other two civilians were killed in strikes on the Idlib province villages of Kansafra and Khan al-Subul, the Britain-based monitor said.

The September deal signed by Russia and rebel backer Turkey was supposed to set up a buffer zone around the Idlib region, but it was never fully implemented as the jihadists refused to pull back from the front lines.

Hostilities deepened In January when Hayat Tahrir al-Sham -- an alliance led by Syria's former Al-Qaeda affiliate -- took over administrative control of the region.

Since late April, more than 460 civilians have been killed in government or Russian bombardment, according to the Observatory.

The violence has forced around 330,000 people to flee their homes and hit 23 health centres, the United Nations says.

The war in Syria has killed more than 370,000 people and displaced millions since it erupted in 2011 with the brutal repression of protests against President Bashar al-Assad.